Pulse generator

ABSTRACT

At an occasion of a level transition when a second periodic voltage becomes equal to a main reference voltage a first periodic voltage generating circuit starts a first monotonically changing time-period in which a voltage value of a first periodic voltage increases monotonically from 0, which is an initial value, towards a voltage value of the main reference voltage. At an occasion of a level transition of a first main switching signal when the first periodic voltage becomes equal to the main reference voltage, a second periodic voltage generating circuit starts a second monotonically changing time-period in which a voltage value of the second periodic voltage increases monotonically from 0, which is an initial value, towards a voltage value of the main reference voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to technology for generating a periodicpulse signal.

2. Description of the Related Art

The oscillator circuit of Patent Document 1 is configured such that,after charging a capacitor from a ground potential to a referencepotential, a discharge switch is turned ON and the capacitor isdischarged to return from the reference potential to the groundpotential, and a signal, which has time required for this sequence ofcharging and discharging as one period, is outputted.

Patent Document 1: Japanese Patent Application, Laid-Open No. S62-272616

An ON resistance of a switch configured as a MOSFET (Metal OxideSemiconductor Field-Effect Transistor) or the like changes according tousage environment thereof. When the ON resistance of the switch changes,discharge time of the capacitor also changes. In the technology ofPatent Document 1, the change in the discharge time directly becomes anerror of the period of the output signal. When high accuracy is requiredin the period that it is as small as possible.

SUMMARY OF THE INVENTION

The present invention has been carried out in recognition of thissituation, and a general purpose thereof is to provide technology forraising accuracy of a period of a pulse signal.

An embodiment of the present invention is a pulse generator. The pulsegenerator is provided with a periodic voltage generator which generatesa plurality of periodic voltages, and a pulse signal output unit whichoutputs a pulse signal having a period determined in accordance with theplurality of periodic voltages. The periodic voltage generator generatesthe plurality of periodic voltages which periodically repeat respectivemonotonically changing time-periods in which a voltage value changesmonotonically from an initial value towards a target value, and at anoccasion of termination of a monotonically changing time-period of acertain periodic voltage, starts a monotonically changing time-period ofanother periodic voltage. The pulse signal output unit uses voltage ofthe monotonically changing time-period of at least one periodic voltageto set an edge of the pulse signal.

According to this embodiment, a period of a pulse signal is not affectedby the monotonically changing time-period ending and the time until thevoltage value of the periodic voltage returns to an initial value.Therefore even if this time changes due to usage environment or the likeof a pulse generator, this change is prevented from causing an error inthe period of the pulse signal, and accuracy of the period of the pulsesignal is raised.

Another embodiment of the present invention is a pulse generator. Thepulse generator is provided with a first periodic voltage generatingcircuit which charges and discharges a first capacitor and generates afirst periodic voltage that periodically repeats a first monotonicallychanging time-period in which a voltage value changes monotonically froma first initial value towards a first main target value; a first maincomparator which compares a voltage value of the first periodic voltagewith the first main target value, and generates a first main switchingsignal that makes a level transition when these values are equal; asecond periodic voltage generating circuit which charges and dischargesa second capacitor and generates a second periodic voltage thatperiodically repeats a second monotonically changing time-period inwhich a voltage value changes monotonically from a second initial valuetowards a second main target value; a second main comparator whichcompares a voltage value of the second periodic voltage with the secondmain target value, and generates a second main switching signal thatmakes a level transition when these values are equal; and a pulse outputcircuit which uses the level transition of the first main switchingsignal and the second main switching signal to output a pulse signal inwhich an edge appears when the voltage value of the first periodicvoltage reaches the first main target value from the first initial valueand the voltage value of the second periodic voltage reaches the secondmain target value from the second initial value. The first periodicvoltage generating circuit uses the level transition of the second mainswitching signal that occurs when the voltage value of the secondperiodic voltage reaches the second main target value from the secondinitial value, to start the first monotonically changing time-period,and the second periodic voltage generating circuit uses the leveltransition of the first main switching signal that occurs when thevoltage value of the first periodic voltage reaches the first maintarget value from the first initial value, to start the secondmonotonically changing time-period.

According to this embodiment, since the period of the pulse signaloutputted from the pulse output circuit is determined according to thelength of the first monotonically changing time-period and the secondmonotonically changing time-period, the time from when the firstmonotonically changing time-period ends until the voltage value of thefirst periodic voltage returns to the first initial value, and the timefrom when the second monotonically changing time-period ends until thevoltage value of the second periodic voltage returns to the secondinitial value does not affect the period of the pulse signal. Thus, evenif these times change due to usage environment or the like of the pulsegenerator, the change is prevented from causing an error in the periodof the pulse signal, and the accuracy of the period of the pulse signalis raised.

A further embodiment of the present invention is an electronic device.The electronic device is provided with the abovementioned pulsegenerator and a circuit which operates with output of the pulsegenerator as a clock.

According to this embodiment, since the accuracy of the period of thepulse signal used as the clock is high, performance of the electronicdevice can be raised.

An even further embodiment of the present invention is a pulsegeneration method of generating a periodic pulse signal. This methodrepeats steps of: charging or discharging a first capacitor, andmonotonically changing a voltage value thereof from a first initialvalue towards a first main target value; comparing a voltage amount ofthe first capacitor with the first main target value, and generating afirst main switching signal that makes a level transition when theselevels become equal; using the level transition of the first mainswitching signal to set an edge of the pulse signal at timing at whichthe amount of the voltage of the first capacitor reaches the first maintarget value from the first initial value, and also starting charging ordischarging a second capacitor; charging or discharging the secondcapacitor, and monotonically changing a voltage value thereof from asecond initial value towards a second main target value; comparing avoltage amount of the second capacitor with the second main target valueand generating a second main switching signal that makes a leveltransition when these levels become equal; and using the leveltransition of the second main switching signal to set an edge of thepulse signal at timing at which the amount of the voltage of the secondcapacitor reaches the second main target value from the second initialvalue, and also starting charging or discharging the first capacitor.

It is to be noted that any arbitrary combination or rearrangement of theabove-described structural components and so forth is effective as andencompassed by the present embodiments.

Moreover, this summary of the invention does not necessarily describeall necessary features so that the invention may also be asub-combination of these described features.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying drawings which are meant to be exemplary,not limiting, and wherein like elements are numbered alike in severalFigures, in which:

FIG. 1 is a circuit diagram showing a configuration of a pulse generatoraccording to an embodiment;

FIG. 2 is a time chart showing operation of the pulse generator shown inFIG. 1; and

FIG. 3 is a circuit diagram showing a configuration of a pulse generatoraccording to a comparative example.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described based on preferred embodiments whichdo not intend to limit the scope of the present invention but exemplifythe invention. All of the features and the combinations thereofdescribed in the embodiment are not necessarily essential to theinvention.

In the present specification, a state in which “member A is connected tomember B” includes cases in which the member A and the member B aredirectly and physically connected, and cases in which the member A andthe member B are indirectly connected via another member that does notaffect an electrical connection state.

In a similar way, “a state in which member C is arranged between memberA and member B” includes, in addition to cases in which the member A andthe member C, or the member B and the member C are directly connected,cases in which the members are indirectly connected via another memberthat does not affect an electrical connection state.

FIG. 1 shows a configuration of a pulse generator 100 according to theembodiment. The pulse generator 100 can be used as a clock generationsource used, for example, in a motor driver or the like, which drives amotor for a zoom lens, auto-focusing, or an aperture of a digitalcamera.

The pulse generator 100 is provided with a periodic voltage generator 52and a pulse signal output unit 54, and is monolithically integrated onone semiconductor substrate together with another circuit block such asa motor driver or the like. Furthermore, “monolithically integrated”includes cases in which all component elements of the circuit are formedon the semiconductor substrate, and cases in which main componentelements of the circuit are integrated, and some resistors or the likefor adjusting a circuit constant may be arranged outside thesemiconductor substrate. By monolithically integrating on onesemiconductor substrate, installation in the electronic device becomeseasier.

The periodic voltage generator 52 generates a first periodic voltageVsaw1 and a second periodic voltage Vsaw2. The pulse signal output unit54 outputs a pulse signal Spls whose period is determined according tothe first periodic voltage Vsaw1 and the second periodic voltage Vsaw2.

The periodic voltage generator 52 has a first periodic voltagegenerating circuit 74 and a second periodic voltage generating circuit76.

The first periodic voltage generating circuit 74 has a first capacitorC1, a first constant current source 84, and a first switch M1, andoutputs voltage of the first capacitor C1 as the first periodic voltageVsaw1. The first constant current source 84 and the first capacitor C1are connected in series between a power supply terminal and a groundterminal, and the first switch M1 is arranged between a connection pointthereof and a ground terminal. When the first switch M1 is OFF, thefirst capacitor C1 is charged by a current I from the first constantcurrent source 84. When the first switch M1 is ON, the first capacitorC1 is discharged, and a voltage value thereof is initialized to 0.

The second periodic voltage generating circuit 76 has a second capacitorC2, a second constant current source 86, and a second switch M2, andoutputs voltage of the second capacitor C2 as the second periodicvoltage Vsaw2. The second constant current source 86 and the secondcapacitor C2 are connected in series between a power supply terminal anda ground terminal, and the second switch M2 is arranged between aconnection point thereof and a ground terminal. When the second switchM2 is OFF, the second capacitor C2 is charged by a current I from thesecond constant current source 86. When the second switch M2 is ON, thesecond capacitor C2 is discharged, and a voltage value thereof isinitialized to 0.

The first constant current source 84 and the second constant currentsource 86 are formed using a well-known current mirror circuit, andcurrent values thereof become equal. Furthermore, the first capacitor C1and the second capacitor C2 are configured so that capacitance valuesthereof are equal. The first switch M1 and the second switch M2 areelectronic switches implemented by a MOSFET, for example, and in thepresent embodiment are configured as N-channel MOSFETs.

A first switch control signal Vm1 that performs ON-OFF control of thefirst switch M1 is obtained by passing a second main switching signalS21 and a second sub-switching signal S22, described later, through awell known logic gate, which is not shown in the figures. A logicalexpression of the first switch control signal Vm1 is represented as (S21XOR S22) or (S21 AND *S22).

In a similar way, a second switch control signal Vm2 that performsON-OFF control of the second switch M2 is obtained by passing a firstmain switching signal S11, described later, and a first sub-switchingsignal S12, through a well known logic gate, which is not shown in thefigures. A logical expression of the second switch control signal Vm2 isrepresented as (S11 XOR S12) or (S11 AND *S12).

The pulse signal output unit 54 is provided with a first main comparatorCMP11, a first sub-comparator CMP12, a second main comparator CMP21, asecond sub-comparator CMP22, and an output circuit 94.

The first periodic voltage Vsaw1 is inputted to an inverting inputterminal of the first main comparator CMP11, and a main referencevoltage Vmain is inputted to a non-inverting input terminal. The firstperiodic voltage Vsaw1 is inputted to an inverting input terminal of thefirst sub-comparator CMP12, and a sub-reference voltage Vsub is inputtedto a non-inverting input terminal. Furthermore, the voltage value of thesub-reference voltage Vsub is between 0 and the voltage value of themain reference voltage Vmain. That is, a relationship 0<Vsub<Vmain isestablished.

The second periodic voltage Vsaw2 is inputted to an inverting inputterminal of the second main comparator CMP21, and the main referencevoltage Vmain is inputted to a non-inverting input terminal. The secondperiodic voltage Vsaw2 is inputted to an inverting input terminal of thesecond sub-comparator CMP22, and the sub-reference voltage Vsub isinputted to a non-inverting input terminal.

The first main comparator CMP11 compares the voltage value of the mainreference voltage Vmain and the voltage value of the first periodicvoltage Vsaw1, and generates the first main switching signal S11 thatmakes a level transition when these levels become equal. The firstsub-comparator CMP12 compares the voltage value of the sub-referencevoltage Vsub and the voltage value of the first periodic voltage Vsaw1,and generates the first sub-switching signal S12 that makes a leveltransition when these levels become equal.

The second main comparator CMP21 compares the voltage value of the mainreference voltage Vmain and the voltage value of the second periodicvoltage Vsaw2, and generates the second main switching signal S21 thatmakes a level transition when these voltage values become equal. Thesecond sub-comparator CMP22 compares the voltage value of thesub-reference voltage Vsub and the voltage value of the second periodicvoltage Vsaw2, and generates the second sub-switching signal S22 thatmakes a level transition when these levels become equal.

The output circuit 94 outputs a pulse signal Spls based on the firstmain switching signal S11 and the first sub-switching signal S12, andthe second main switching signal S21 and the second sub-switching signalS22. The pulse signal Spls is the final output of the pulse generator100. Furthermore, the output circuit 94 is configured by combiningwell-known logic gates.

Below, operation of the pulse generator 100 is explained by a time chartwhile referring to FIG. 1.

FIG. 2 is a time chart showing the operation of the pulse generator 100shown in FIG. 1. The time chart shows, in order, from the top, the firstperiodic voltage Vsaw1, the first main switching signal S1, the firstsub-switching signal S12, ON-OFF status of the first switch M1, thesecond periodic voltage Vsaw2, the second main switching signal S21, thesecond sub-switching signal S22, ON-OFF status of the second switch M2,and the pulse signal Spls.

The first periodic voltage generating circuit 74 turns the first switchM1 OFF, and charges the first capacitor C1, whose voltage is initializedto 0, by the first constant current source 84. In this way, a firstmonotonically changing time-period T1 is started, in which the voltagevalue of the first periodic voltage Vsaw1 increases monotonically froman initial value of 0 towards the voltage value of the main referencevoltage Vmain.

When the first periodic voltage Vsaw1 becomes equal to the sub-referencevoltage Vsub, the first sub-switching signal S12 transits from a highlevel to a low level. The output circuit 94 sets a positive edge of thepulse signal Spls at timing of this level transition. In addition, thesecond periodic voltage generating circuit 76 turns the second switch M2ON at an occasion of this level transition, and initializes the voltagevalue of the second capacitor C2 to 0. Together with the voltage valueof the second capacitor C2 being initialized to 0, the second mainswitching signal S21 and the second sub-switching signal S22 go to ahigh level.

When the first periodic voltage Vsaw1 becomes equal to the mainreference voltage Vmain, the first main switching signal S11 transitsfrom a high level to a low level. The output circuit 94 sets a negativeedge of the pulse signal Spls at timing of this level transition. Inaddition, the second periodic voltage generating circuit 76 turns thesecond switch M2 OFF at an occasion of this level transition, andcharges the second capacitor C2 by the second constant current source86. In this way, a second monotonically changing time-period T2 isstarted, in which the voltage value of the second periodic voltage Vsaw2increases monotonically from an initial value of 0 towards the voltagevalue of the main reference voltage Vmain. Furthermore, the first mainswitching signal S11 transiting from a high level to a low level meansthe end of the first monotonically changing time-period T1. That is, thesecond periodic voltage generating circuit 76 uses the level transitionof the first main switching signal S11 that occurs when the voltagevalue of the first periodic voltage Vsaw1 reaches the voltage value ofthe main reference voltage Vmain from the initial value, to start thesecond monotonically changing time-period T2, at an occasion of theending of the first monotonically changing time-period T1.

When the second periodic voltage Vsaw2 becomes equal to thesub-reference voltage Vsub, the second sub-switching signal S22 transitsfrom a high level to a low level. The output circuit 94 sets a positiveedge of the pulse signal Spls at timing of this level transition.Furthermore, the first periodic voltage generating circuit 74 turns thefirst switch M1 ON at an occasion of this level transition, andinitializes the voltage value of the first capacitor C1 to 0. Togetherwith the voltage value of the first capacitor C1 being initialized to 0,the first main switching signal S11 and the first sub-switching signalS12 go to a high level.

When the second periodic voltage Vsaw2 becomes equal to the mainreference voltage Vmain, the second main switching signal S21 transitsfrom a high level to a low level. The output circuit 94 sets a negativeedge of the pulse signal Spls at timing of this level transition.Furthermore, the first periodic voltage generating circuit 74 turns thefirst switch M1 OFF at an occasion of this level transition, and chargesthe first capacitor C1 by the first constant current source 84. In thisway the first monotonically changing time-period T1 of a subsequentcycle starts. Furthermore, the second main switching signal S21transiting from a high level to a low level means the end of the secondmonotonically changing time-period T2. That is, the first periodicvoltage generating circuit 74 uses the level transition of the secondmain switching signal S21 that occurs when the voltage value of thesecond periodic voltage Vsaw2 reaches the voltage value of the mainreference voltage Vmain from the initial value, to start the firstmonotonically changing time-period T1, at an occasion of the ending ofthe second monotonically changing time-period T2.

By repeating the above operations, the first monotonically changingtime-period T1 and the second monotonically changing time-period T2alternately repeat. Furthermore, since current values of the firstconstant current source 84 and the second constant current source 86 areequal and capacitance values of the first capacitor C1 and the secondcapacitor C2 are equal, lengths of the first monotonically changingtime-period T1 and the second monotonically changing time-period T2 areequal. This length is one period of the pulse signal Spls.

In the pulse generator 100 of the present embodiment it should be notedthat the length of the initialization time-period (referred to below as“initialization time”), in which the first capacitor C1 and the secondcapacitor C2 are discharged and voltage values thereof are initializedto 0, does not affect the period of the pulse signal Spls. That is,since the period of the pulse signal Spls is determined by the lengthsof the first monotonically changing time-period T1 and the secondmonotonically changing time-period T2, the period of the pulse signalSpls is not affected by the initialization time. When the initializationtime changes due to external environment, since in the presentembodiment the occurrence of an error in the period of the pulse signalSpls due to the change in the initialization time is prevented in thisway, the accuracy of the period of the pulse signal Spls is raised.

Furthermore, while the technology of Patent Document 1 takes as oneperiod the time required for a sequence of charging and discharging, inwhich, after charging a capacitor from a ground potential to a referencepotential, the capacitor is discharged from the reference potential backto the ground potential, in the present embodiment since each of thefirst monotonically changing time-period T1 and the second monotonicallychanging time-period T2 is one period, frequency of the pulse signalSpls can be increased.

Below, making reference to a comparative example, an effect of thepresent embodiment is explained from another viewpoint. The comparativeexample shows a case in which a pulse signal is generated using, forexample, a triangular wave generating circuit as described in FIG. 4 ofJapanese Patent Application, Laid-Open No. 2006-20177.

FIG. 3 shows a configuration of a pulse generator 800 according to thecomparative example. In a charging time-period, the pulse generator 800charges a capacitor C by a charging constant current source 884A, with adischarging constant current source 884B in an OFF state, and when thevoltage value of the capacitor C becomes equal to a voltage value of ahigh potential reference voltage VrefH, moves to a dischargingtime-period; and in a discharging time-period, the pulse generator 800discharges the capacitor C, with the discharging constant current source884B in an ON state, and when the voltage value of the capacitor Cbecomes equal to a voltage value of a low potential reference voltageVrefL, moves to a charging time-period. During repetition of theseoperations, each time the voltage value of the capacitor C becomes equalto the voltage value of the high potential reference voltage VrefH andto the low potential reference voltage VrefL, an output circuitgenerates a pulse signal having an edge. Furthermore, the chargingconstant current source 884A and the discharging constant current source884B are configured as current mirror circuits. Since a current value ofthe charging constant current source 884A is I, and a current value ofthe discharging constant current source 884B is 2I, a total currentvalue of a discharging current of the capacitor C in a dischargingtime-period is I.

When the discharging constant current source 884B is turned ON when thedischarging time-period is started, a transient type of current (below,referred to as “transient current”) at this start-up time changes due toexternal environment. The change in the transient current causes anerror in a period of an output pulse signal of the pulse generator 800.With regard to this point, in the pulse generator 100 of the presentembodiment, since turning ON and OFF a current source by providing twoperiodic voltage generating circuits for determining the period of thepulse signal Spls is not necessary, occurrence of an error in the periodof the pulse signal Spls caused by the change in the transient currentis prevented, and accuracy is raised.

Furthermore, in the pulse generator 800 of the comparative example, ifresponse speed of a comparator is changed by the external environment,the voltage value of the capacitor changes at both starting and endingof the charging time-period. In this way, the length of the chargingtime-period and the discharging time-period changes, and an error occursin the period of the output pulse signal of the pulse generator 800. Incomparison to this, in the pulse generator 100 of the present embodimentthe voltage value, when charging of the first capacitor C1 and thesecond capacitor C2 is started, is initialized to a ground potential,that is, a stable fixed potential. That is, since the voltage values ofthe first periodic voltage Vsaw1 and the second periodic voltage Vsaw2are relatively stable at the start of the first monotonically changingtime-period T1 and the second monotonically changing time-period T2, apulse signal having a period of higher accuracy than the pulse generator800 of the comparative example can be generated.

The abovementioned embodiment is an example, and a person skilled in theart will understand that various modifications in combinations ofvarious component elements and various processes thereof are possible,and that such modified examples are within the scope of the presentinvention. Below, modified examples are listed.

Modified Example

In the embodiment described above, each of the lengths of the firstmonotonically changing time-period T1 and the second monotonicallychanging time-period T2, which have equal lengths, is taken as oneperiod of the pulse signal Spls. In a modified example, the total of thelengths of the first monotonically changing time-period T1 and thesecond monotonically changing time-period T2 is taken as one period ofthe pulse signal Spls. In such a case, the output circuit 94 does notuse a level transition of the first sub-switching signal S12 and a leveltransition of the second sub-switching signal S22 in setting an edge ofthe pulse signal Spls, but sets a positive edge of the pulse signal Splsat timing at which the first main switching signal S11 transits from ahigh level to a low level, and sets a negative edge of the pulse signalSpls at timing at which the second main switching signal S21 transitsfrom a high level to a low level.

The present modified example also achieves an effect similar to theembodiment. Additionally, according to the present modified example,consideration need not be particularly given to making capacitancevalues of the first capacitor C1 and the second capacitor C2 equal, orto making current values of the first constant current source 84 and thesecond constant current source 86 equal. That is, when each of thelengths of the first monotonically changing time-period T1 and thesecond monotonically changing time-period T2 is taken as one period ofthe pulse signal Spls, as in the embodiment, consideration must be givento making the lengths of the first monotonically changing time-period T1and the second monotonically changing time-period T2 equal, but in thepresent modified example the total of the lengths of the firstmonotonically changing time-period T1 and the second monotonicallychanging time-period T2 is taken as one period of the pulse signal Spls,so that such consideration is unnecessary. In this way, the pulsegenerator 100 is easy to manufacture.

Furthermore, since even if the lengths of the first monotonicallychanging time-period T1 and the second monotonically changingtime-period T2 in the present modified example are different, this doesnot cause an error in the period of the pulse signal Spls, and it ispossible to raise the accuracy of the period of the pulse signal Splseven further than in the embodiment. However, in the present modifiedexample, the frequency of the pulse signal Spls is half that of cases ofthe embodiment. Usage of either the embodiment or the present modifiedexample may be changed as appropriate according to required frequency oraccuracy. According to the present type of modified example, applicationscope of the pulse generator 100 is widened.

Other Modified Examples

In the embodiment, two periodic voltage generating circuits fordetermining the period of the pulse signal Spls were provided, but thereis no limitation thereto, and three or more periodic voltage generatingcircuits may be provided. An effect similar to the embodiment is alsoobtained in such cases.

In the embodiment, two comparators are provided for each of the firstperiodic voltage Vsaw1 and the second periodic voltage Vsaw2, but thereis no limitation thereto, and three comparators may be provided for eachof the first periodic voltage Vsaw1 and the second periodic voltageVsaw2. Specifically, two other sub-comparators are additionallyprovided, inputting a sub-reference voltage other than the mainreference voltage Vmain and the sub-reference voltage Vsub to anon-inverting input terminal, and inputting each of the first periodicvoltage Vsaw1 and the second periodic voltage Vsaw2 to a non-invertinginput terminal. The output circuit 94 also uses level transitions ofoutput signals of these other two sub-comparators in setting edges ofthe pulse signal Spls. According to this, it is possible to increase thefrequency of the pulse signal Spls. In the same way, by furtherincreasing the number of comparators, it is also possible to furtherincrease the frequency of the pulse signal Spls.

In the embodiment, a first sub-comparator CMP12 and a secondsub-comparator CMP22 are configured based on a dual input differentialamplifier, but alternatively an inverter may be used. In such cases, theinverter is configured such that there is a transition in output level,with input between 0 and the voltage value of the main reference voltageVmain. That is, a threshold of the inverter is used as the sub-referencevoltage Vsub. In cases in which accuracy is not required in duty ratioof the pulse signal Spls, by configuring the first sub-comparator CMP12and the second sub-comparator CMP22 as inverters as in the presentmodified example, it is possible to reduce circuit scale of the pulsegenerator 100.

Instead of the first sub-comparator CMP12 of the embodiment, a timer maybe provided which, at an occasion at which the second monotonicallychanging time-period T2 ends, measures a time shorter than the length ofthe first monotonically changing time-period T1, and when this time haspassed, there is a transition in output level. By using this leveltransition, an effect similar to the embodiment is obtained. The samemay be said concerning the second sub-comparator CMP22.

In the embodiment, cases in which the initial values of the voltagevalues of the first periodic voltage Vsaw1 and the second periodicvoltage Vsaw2 are smaller than the voltage value of the main referencevoltage Vmain are illustrated by example, but there is no limitationthereto, and the size relationships thereof may be reversed. That is,the first monotonically changing time-period T1 and the secondmonotonically changing time-period T2 may be time-periods in which eachof the voltage values of the first periodic voltage Vsaw1 and the secondperiodic voltage Vsaw2 decrease monotonically from the initial valuetowards the voltage value of the main reference voltage Vmain.

In the embodiment, the first switch M1 and the second switch M2 areprovided to initialize the voltage values of the first periodic voltageVsaw1 and the second periodic voltage Vsaw2, but there is no limitationthereto, and a discharging current source may be used in place thereof.In such cases, by setting to the initial value the voltage value of eachof the first capacitor C1 and the second capacitor C2 by the time ofstarting each of the first monotonically changing time-period T1 and thesecond monotonically changing time-period T2, an effect similar to theembodiment can be achieved.

In the embodiment, the respective first capacitor C1 and secondcapacitor C2 are charged by the respective first constant current source84 and second constant current source 86, but there is no limitationthereto, and instead of the first constant current source 84 and thesecond constant current source 86, a first resistor and a secondresistor may be used and these resistors may be operated as currentsources. In such cases, it is possible to achieve an effect similar tothe embodiment.

In the embodiment, the voltage values of each of the first periodicvoltage Vsaw1 and the second periodic voltage Vsaw2 are compared withthe voltage value of the main reference voltage Vmain by the first maincomparator CMP11 and the second main comparator CMP21. In the modifiedexample, functions of the first main comparator CMP11 and the secondmain comparator CMP21 may be realized by sharing one comparator by timedivision (below, this one comparator is referred to as a “sharedcomparator” or “common comparator”). In such cases, a selector isprovided to select either of the first periodic voltage Vsaw1 and thesecond periodic voltage Vsaw2, and the selected voltage is inputted toan inverting input terminal of the shared comparator. The selectorswitches input to the shared comparator from the first periodic voltageVsaw1 to the second periodic voltage Vsaw2 at an occasion at which thefirst monotonically changing time-period T1 ends, and switches input tothe shared comparator from the second periodic voltage Vsaw2 to thefirst periodic voltage Vsaw1, at an occasion at which the secondmonotonically changing time-period T2 ends. In this way, the selectoralternately inputs the first periodic voltage Vsaw1 and the secondperiodic voltage Vsaw2 to the shared comparator. According to thepresent modified example, circuit scale can be reduced by reducing thenumber of comparators. Functions of the first sub-comparator CMP12 andthe second sub-comparator CMP22 can be similarly realized by sharing onecomparator by time division.

Furthermore, setting of a logical value for the high level and the lowlevel described in the embodiment is one example, and it is possible tofreely make modifications by appropriate inversion by an inverter or thelike.

While the preferred embodiments of the present invention have beendescribed using specific terms, such description is for illustrativepurposes only, and it is to be understood that changes and variationsmay be made without departing from the spirit or scope of the appendedclaims.

1. A pulse generator comprising: a periodic voltage generator whichgenerates a plurality of periodic voltages; and a pulse signal outputunit which outputs a pulse signal having a period determined inaccordance with the plurality of periodic voltages; wherein the periodicvoltage generator generates the plurality of periodic voltages whichperiodically repeat respective monotonically changing time-periods inwhich a voltage value changes monotonically from an initial valuetowards a target value, and at an occasion at which a monotonicallychanging time-period of a certain periodic voltage ends, starts amonotonically changing time-period of another periodic voltage, and thepulse signal output unit uses voltage of the monotonically changingtime-period of at least one periodic voltage to set an edge of the pulsesignal.
 2. The pulse generator according to claim 1, wherein theperiodic voltage generator comprises a plurality of periodic voltagegenerating circuits which respectively generate the plurality ofperiodic voltages; each of the plurality of periodic voltage generatingcircuits comprises: a capacitor which outputs the periodic voltage, acurrent source which charges or discharges the capacitor, and a switchwhich sets a voltage value of the capacitor to an initial value; andfrom a state in which the voltage value is set to the initial value bythe switch, the monotonically changing time-period is started bycharging or discharging the capacitor with the current source.
 3. Thepulse generator according to claim 1, wherein the pulse signal outputunit comprises a signal generating circuit which generates a switchingsignal that makes a level transition when a voltage level of each of theplurality of periodic voltages reaches a target value from an initialvalue; and the periodic voltage generator starts a monotonicallychanging time-period of another periodic voltage, at an occasion of alevel transition of the switching signal, when a voltage value of acertain periodic voltage reaches a target value from an initial value.4. The pulse generator according to claim 3, wherein the pulse signaloutput unit uses a level transition of the switching signal to output apulse signal in which an edge appears every time the voltage value ofthe respective plurality of periodic voltages reaches a target valuefrom an initial value.
 5. A pulse generator comprising: a first periodicvoltage generating circuit which charges and discharges a firstcapacitor and generates a first periodic voltage that periodicallyrepeats a first monotonically changing time-period in which a voltagevalue changes monotonically from a first initial value towards a firstmain target value; a first main comparator which compares a voltagevalue of the first periodic voltage with the first main target value,and generates a first main switching signal that makes a leveltransition when these values are equal; a second periodic voltagegenerating circuit which charges and discharges a second capacitor andgenerates a second periodic voltage that periodically repeats a secondmonotonically changing time-period in which a voltage value changesmonotonically from a second initial value towards a second main targetvalue; a second main comparator which compares a voltage value of thesecond periodic voltage with the second main target value, and generatesa second main switching signal that makes a level transition when thesevalues are equal; and a pulse output circuit which uses the leveltransition of the first main switching signal and the second mainswitching signal to output a pulse signal in which an edge appears whenthe voltage value of the first periodic voltage reaches the first maintarget value from the first initial value and the voltage value of thesecond periodic voltage reaches the second main target value from thesecond initial value; wherein the first periodic voltage generatingcircuit uses the level transition of the second main switching signalthat occurs when the voltage value of the second periodic voltagereaches the second main target value from the second initial value, tostart the first monotonically changing time-period, and the secondperiodic voltage generating circuit uses the level transition of thefirst main switching signal that occurs when the voltage value of thefirst periodic voltage reaches the first main target value from thefirst initial value, to start the second monotonically changingtime-period.
 6. The pulse generator according to claim 5, furthercomprising: a first sub-comparator which compares the voltage value ofthe first periodic voltage with a first sub-target value between thefirst initial value and the first main target value, and generates afirst sub-switching signal that makes a level transition when thesevalues are equal; and a second sub-comparator which compares the voltagevalue of the second periodic voltage with a second sub-target valuebetween the second initial value and the second main target value, andgenerates a second sub-switching signal that makes a level transitionwhen these values are equal; wherein the first periodic voltagegenerating circuit and the second periodic voltage generating circuitare configured such that the first monotonically changing time-periodand the second monotonically changing time-period have equal lengths,and the pulse output circuit uses the level transitions of the firstsub-switching signal and the second sub-switching signal to output apulse signal in which an edge appears when the voltage value of thefirst periodic voltage reaches the first sub-target value from the firstinitial value and when the voltage value of the second periodic voltagereaches the second sub-target value from the second initial value. 7.The pulse generator according to claim 6, wherein the first periodicvoltage generating circuit uses the level transition of the secondsub-switching signal to set a voltage value of the first capacitor to aninitial value, at an occasion at which the voltage value of the secondperiodic voltage reaches the second sub-target value from the secondinitial value; and the second periodic voltage generating circuit usesthe level transition of the first sub-switching signal to set a voltagevalue of the second capacitor to an initial value, at an occasion atwhich the voltage value of the first periodic voltage reaches the firstsub-target value from the first initial value.
 8. The pulse generatoraccording to claim 1, wherein the pulse generator is monolithicallyintegrated on one semiconductor substrate.
 9. The pulse generatoraccording to claim 5 wherein the pulse generator is monolithicallyintegrated on one semiconductor substrate.
 10. An electronic devicecomprising: the pulse generator according to claim 1; and a circuitwhich operates with output of the pulse generator as a clock.
 11. Anelectronic device comprising: the pulse generator according to claim 5;and a circuit which operates with output of the pulse generator as aclock.
 12. A pulse generation method of generating a periodic pulsesignal, the method comprising repeating the steps of: charging ordischarging a first capacitor, and monotonically changing a voltagevalue thereof from a first initial value towards a first main targetvalue; comparing a voltage value of the first capacitor with the firstmain target value and generating a first main switching signal thatmakes a level transition when these values become equal; using the leveltransition of the first main switching signal to set an edge of thepulse signal at timing at which the voltage value of the first capacitorreaches the first main target value from the first initial value, andalso starting charging or discharging a second capacitor; charging ordischarging the second capacitor, and monotonically changing a voltagevalue thereof from a second initial value towards a second main targetvalue; comparing a voltage value of the second capacitor with the secondmain target value and generating a second main switching signal thatmakes a level transition when these values become equal; and using thelevel transition of the second main switching signal to set an edge ofthe pulse signal at timing at which the voltage value of the secondcapacitor reaches the second main target value from the second initialvalue, and also starting charging or discharging the first capacitor.13. The pulse generation method according to claim 12 wherein lengths ofa first monotonically changing time-period in which a voltage value ofthe first capacitor reaches the first main target value from the firstinitial value and a second monotonically changing time-period in which avoltage value of the second capacitor reaches the second main targetvalue from the second initial value are equal, the method comprisingrepeating the further steps of: comparing a voltage value of the firstcapacitor with a first sub-target value between the first initial valueand the first main target value, and generating a first sub-switchingsignal that makes a level transition when these values become equal;using the level transition of the first sub-switching signal to set anedge of the pulse signal at timing at which the voltage value of thefirst capacitor reaches the first sub-target value from the firstinitial value; comparing a voltage value of the second capacitor with asecond sub-target value between the second initial value and the secondmain target value, and generating a second sub-switching signal thatmakes a level transition when these values become equal; and using thelevel transition of the second sub-switching signal to set an edge ofthe pulse signal at timing at which the voltage value of the secondcapacitor reaches the second sub-target value from the second initialvalue.